The objective of the project is to find physiological correlates to account for the loss of cochlear sensitivity during the initial stages of noise overexposure. The endocochlear potential, the tone-induced cochlear responses and the potassium activities of the endolymph and perilymph have been recorded in vivo from anesthetized guinea pigs. Changes in these parameters have been studied during repeated, discrete periods of acoustic overstimulation. When overstimulated with broad band white noise, although the cochlear microphonics and action potentials were suppressed the endolymph and perilymph potassium concentrations were maintained at their normal values. The endocochlear potential was found in most cases to be increased by acoustic overstimulation.